You won’t find “PlayerUnknown’s Battlegrounds” on any of The NPD Group’s monthly video game sales charts. It’s not available at GameStop. And it, officially, won’t be finished until the end of the year. (In fact, it’s quite buggy and has fairly shoddy graphics.)

But that hasn’t stopped the online action game from selling nearly 5 million copies and making over $100 million in revenue for its South Korean maker, Bluehole, in the past four months.

And it hasn’t stopped Microsoft from using the game’s massive and sudden popularity to try to get an edge over its perennial rival, Sony.

“PlayerUnknown’s Battlegrounds” has amassed those sales and that enviable player base via the Steam digital distribution system. Taking a page from the successful playbook of “Minecraft” and other big independent titles, it released a playable, but incomplete, version of the game via Steam’s “Early Access” program, charging $30 per copy.

Lots of developers do that, usually to help fund the creation of their games, but few see their efforts go as viral as “PlayerUnknown’s Battlegrounds” has. Fans love it — and tell their friends, who buy a copy and further spread the word. Even celebrities, such as Terry Crews, are vocal about their love of the game.

To put the popularity of “PlayerUnknown’s Battlegrounds” into perspective, it

Take-Two Interactive Software‘s “Grand Theft Auto V” for peak number of concurrent users — with 382,561 people playing at once versus 360,761 playing “GTA” in April 2015.

So what makes the game so popular?

“They’ve differentiated the multiplayer experience from ‘Call of Duty,'” says Mike Hickey of Benchmark. “It’s fresh and new and an exciting way to play.”

It works like this: As many as 100 players parachute onto an island and begin scavenging for weapons and equipment to take out their opponents. As the match goes on, the size of the game’s world shrinks, forcing players to battle each other, rather than hiding and winning the game without effort. The last player or team standing wins.

It’s fast-paced and a lot of fun, even for beginners, since every round offers a reset. And its popularity has caught the eye of larger publishers.

And this is where Microsoft comes in. In June, the company said “PlayerUnknown’s Battlegrounds” would be coming to its Xbox One console. This is not happening for Sony’s PlayStation 4 — at least for now — since Microsoft has a limited exclusivity deal with the company. It’s unclear how long that deal lasts, or whether Sony will pursue a partnership with the game’s developer once that exclusivity ends. (Sony did not respond to a request for comment.)

While the Xbox hasn’t sold nearly as many units as PlayStation this generation, there’s still a sizable new audience that will be exposed to the game later this year. And by going multiplatform, “PlayerUnknown’s Battlegrounds” could become a threat to larger publishers’ titles.

“It could very well be that publishers will have to think about acquiring [Bluehole] or altering how they make games,” says Joost van Dreunen, CEO of SuperData.

There is some precedent for a big indie title being swallowed by a larger publisher. Microsoft, in 2014, bought “Minecraft” for $2.5 billion — though that game appealed far beyond the core gamer and had been around for five years.

Before any talk of a buyout happens, though, “PlayerUnknown’s Battlegrounds” will have to prove it has longevity. While it’s certainly off to a roaring start, the key to ensuring long-term customer engagement is regularly offering updates to the game (something Take-Two has successfully done with “Grand Theft Auto Online.”)

That said, Bluehole has effectively used the Early Access model to talk with its community and make regular iterations to the game to increase engagement. (Microsoft has a similar program for console games called Game Preview.)

Those listening skills and the ability to incorporate player requests into the development process while staying true to the developer’s own vision has been the key to the game’s success, say analysts. With each pivot, “PlayerUnknown’s Battlegrounds'” fan base has grown — and there’s no reason to expect that will change anytime in the immediate future.

“It’s not like the team found lightning in a bottle, but it has iterated to create lightning in a bottle,” says Hickey.